DispatcherServlet이란?

Spring MVC에서는 HandlerMapping, HandlerAdapter, ViewResolver 관련 공통된 작업을 DispatcherServlet에서 처리하도록 구현 해놨다. DispatcherServlet에 대해서 알아보자.

DispatcherServlet 서블릿 등록

• DispatcherServlet도 부모 클래스에서 HttpServlet를 상속 받아서 사용하고,  서블릿으로 동작한다.
  • DispatcherServlet -> FrameworkServlet -> HttpServletBean -> HttpServlet
• Spring Boot는 DispatcherServlet을 서블릿으로 자동 등록하면서 모든 경로(urlPatterns="/")에 대해서 매핑한다.
  • 더 자세한 경로가 우선순위가 높다. 그래서 기존에 등록한 서블릿도 함께 동작한다.

요청 흐름

• 서블릿이 호출되면 Httpservlet이 제공하는 service()가 호출된다.
• service()를 시작으로 여러 메서드가 호출되면서 DispatcherServlet.doDispatch()가 호출된다.

@Override
	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		logRequest(request);

		// Keep a snapshot of the request attributes in case of an include,
		// to be able to restore the original attributes after the include.
		Map<String, Object> attributesSnapshot = null;
		if (WebUtils.isIncludeRequest(request)) {
			attributesSnapshot = new HashMap<>();
			Enumeration<?> attrNames = request.getAttributeNames();
			while (attrNames.hasMoreElements()) {
				String attrName = (String) attrNames.nextElement();
				if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
					attributesSnapshot.put(attrName, request.getAttribute(attrName));
				}
			}
		}

		// Make framework objects available to handlers and view objects.
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

		if (this.flashMapManager != null) {
			FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
			if (inputFlashMap != null) {
				request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
			}
			request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
			request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
		}

		RequestPath previousRequestPath = null;
		if (this.parseRequestPath) {
			previousRequestPath = (RequestPath) request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE);
			ServletRequestPathUtils.parseAndCache(request);
		}

		try {
			doDispatch(request, response);
		}
		finally {
			if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
				// Restore the original attribute snapshot, in case of an include.
				if (attributesSnapshot != null) {
					restoreAttributesAfterInclude(request, attributesSnapshot);
				}
			}
			if (this.parseRequestPath) {
				ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
			}
		}
	}

 

doDispatch() 분석

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;

		...

		try {
			ModelAndView mv = null;
			Exception dispatchException = null;

			try {
				...

				// Determine handler for the current request.
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Process last-modified header, if supported by the handler.
				String method = request.getMethod();
				boolean isGet = HttpMethod.GET.matches(method);
				if (isGet || HttpMethod.HEAD.matches(method)) {
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}

				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// Actually invoke the handler.
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}

				applyDefaultViewName(processedRequest, mv);
				mappedHandler.applyPostHandle(processedRequest, response, mv);
			}
			...
            
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		...
	}

위 코드 블럭은 doDispatch() 메서드의 내부이다. doDispatch()는 크게 아래와 같은 역할을 한다.

1. 적절한 HandlerMapping을 선택한다.
2. HandlerAdapter를 선택한다.
3. HandlerAdapter를 통해 Controller를 실행하고 반환값으로 ModelAndView를 받는다.
4. 적절한 view name 생성한다.
5. 생성된 view name으로 ViewResolver를 선택한다.
6. ViewReolsver를 통해 View생성 후 랜더링 한다.

아래 구대의 코드 블럭에서는 ViewResolver가 선택되고 view rendering되는 과정을 나타낸다.

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
			@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
			@Nullable Exception exception) throws Exception {

		...

		// Did the handler return a view to render?
		if (mv != null && !mv.wasCleared()) {
			render(mv, request, response);
			...
		}
		...
	}

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
		// Determine locale for request and apply it to the response.
		Locale locale =
				(this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
		response.setLocale(locale);

		View view;
		String viewName = mv.getViewName();
		if (viewName != null) {
			// We need to resolve the view name.
			view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
			if (view == null) {
				throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
						"' in servlet with name '" + getServletName() + "'");
			}
		}
		else {
			// No need to lookup: the ModelAndView object contains the actual View object.
			view = mv.getView();
			if (view == null) {
				throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
						"View object in servlet with name '" + getServletName() + "'");
			}
		}

		
		...
        
		try {
			...
			view.render(mv.getModelInternal(), request, response);
		}
		...
	}